Poisson naive Bayes classifier (PoissonNB)

Added PoissonNB class to naive_bayes.py to implement a Poisson naieve Bayes classifier.  Poisson classifiers have important applications in systems neuroscience.  See: Ma et Al. *Bayesian inference with probabilistic population codes*  Nat. Neuroscience 9:1432 (2006).

mention here that X has to contain positive values.